1. Field of the Invention
This invention relates to a heat insulating structure for swirl chambers, having hot plugs provided in cavities formed in a cylinder head.
2. Description of the Prior Art
A swirl chamber type engine shown in FIG. 5 has heretofore been known. In the swirl chamber type engine, swirl chambers 52 are formed by two-divided members, i.e. upper members 55 and lower members 54 provided in cavities 58 formed in a cylinder head 53 fixed to a cylinder block 56, as shown in FIG. 5. The upper and lower members 55, 54 are provided with air layers 60 formed between the same members 55, 54 and the cavities 58. The upper members 55 are provided with fuel injection nozzles 57 and glow plugs 59, and the lower members 54 constitute hot plugs having communication ports 61 which communicate the swirl chambers 52 and primary chambers 51, which are formed in cylinders, with each other. In order to fix the upper and lower members 55, 54 in the cavities 58 in the cylinder head 53, metal rings 62 are used. The metal rings 62 are fitted in stepped portions 63 of the cavities in the cylinder head 53 so as to reinforce the lower members 54 by applying compression stress thereto, and also position the same.
A swirl chamber type engine shown in FIG. 6 is also known. In this swirl chamber type engine, swirl chambers 72 are formed by two-divided members, i.e. upper members 75 and lower members 74 provided in cavities 78 formed in a cylinder head 73 fixed to a cylinder block 76, as shown in FIG. 6. The upper and lower members 75, 74 are provided in the cavities 78 with air layers 80 formed between the same members 75, 74 and the cavities 78. The upper members 75 are provided with fuel injection nozzles 77 and glow plugs 79, and the lower members 74 constitute hot plugs having communication ports 81 which communicate the swirl chambers 72 and primary chambers 71, which are formed in cylinders, with each other. Metal rings 70 are fitted around outer circumferential portions of the upper and lower members 75, 74. The metal rings 70 are provided with flange portions 69 formed at lower parts thereof and integrally therewith, and the setting flange portions 69 are press fitted in stepped portions 82 of the cavities in the cylinder head 73 so as to fix the metal rings 70 in the stepped portions 82 of the cavities in the cylinder head 73.
Japanese Utility Model Laid-Open No. 45816/1979 discloses a vortex chamber structure for diesel engines. In this vortex chamber structure for diesel engines, a dome which forms a vortex chamber is fitted in a lower surface of a cylinder head, and a mouthpiece having an ejection port is fixed to this lower surface with an insulating air layer formed between the cylinder head and an outer surface of the dome.
As described above, conventional swirl chambers for diesel engines employ a structure in which mouthpieces are provided between primary combustion chambers and swirl chambers, which mouthpieces are heat insulated. Such swirl chambers as a whole are formed out of a ceramic material, such as silicon nitride into a heat insulating structure, whereby the performance and low-temperature startability of the engine are improved. It has been ascertained that, in order to form swirl chambers into a heat insulating structure, providing air layers around the outer circumferential surfaces of the hot plugs constituting the swirl chambers is effective.